The transmission belts for generators, automobile air conditioners and driving wheels are one important driving mechanism. However, if a transmission belt is too loose, a desired driving effect cannot be attained. A belt's state of tightness is conventionally inspected by an experience technician's visual measurement. Further, is can be detected by pressing the belt by hands, or it can even be detected by listening to the sound when it is operating. However, such measurements based on experience can be easily influenced by the person who does the inspection, therefore being easily distorted by a technician's inexperience and subjectivity. On the other hand, inspecting a transmission belt's tightness is important to driving safety, and suppose that a belt is still valid and a technician retires it, this will lead to a waste of automobile parts and increase car maintenance fee.